Petroleum is generally produced by drilling wellbores through permeable earth formations having petroleum reservoirs therein, and causing petroleum fluids in the reservoir to move to the earth's surface through the wellbores. Movement is accomplished by creating a pressure difference between the reservoir and the wellbore. Produced fluids from the wells may include various quantities of crude oil, natural gas and/or water, depending on the conditions in the particular reservoir being produced. Depending on conditions in the particular reservoir, the amounts and rates at which the various fluids will be extracted from a particular well depend on factors which include pressure difference between the reservoir and the wellbore. As is known in the art, wellbore pressure may be adjusted by operating various devices such as chokes (orifices) disposed in the fluid flow path along the wellbore, pumps, compressors, fluid injection devices (which pump fluid into a reservoir to increase its pressure). Generally speaking, changing the rate at which a total volume of fluid is extracted from any particular wellbore may also affect relative rates at which oil, water and gas are produced from each wellbore.
Production processing equipment, known by a general term “surface facilities”, includes various devices to separate oil and water in liquid form from gas in the produced petroleum. Extracted liquids may be temporarily stored or may be moved to a pipeline for transportation away from the location of the wellbore. Gas may be transported by pipeline to a point of sale, or may be transported by pipe for further processing away from the location of the wellbore. The surface facilities are typically designed to process selected volumes or quantities of produced petroleum. The selected volumes depend on what is believed to be likely volumes of production from various wellbores, and how many wellbores are to be coupled to a particular set of surface facilities. Depending on the physical location of the reservoir, such as below the ocean floor or other remote location, it is often economically advantageous to couple a substantial number of wells, and typically from a plurality of different reservoirs, to a single set of surface facilities. As for less complicated installations, the surface facilities coupled to multiple wells and reservoirs are typically selected to most efficiently process expected quantities of the various fluids produced from the wells. An important aspect of the economic performance of surface facilities is appropriate selection of sizes and capacities of various components of the surface facilities. Equipment which is too small for actual quantities of fluids produced may limit the rate at which the various wellbores may be produced. Such condition may result in poor economic performance of the entire reservoir and surface facility combination. Conversely, equipment which has excess capacity may increase capital costs beyond those necessary, reducing overall rate of return on investment. Still another problem in the efficient use of surface facilities can arise when some wellbores change fluid production rates. As is known in the art, such changes in rate may result from natural depletion of the reservoir, and from unforeseen problems with one or more wellbores in a reservoir, among others. Sometimes, it is possible to change production rates in other wellbores coupled to the surface facilities to maintain throughput in the surface facilities. As is known in the art, however, such production rate changes may be accompanied by changes in relative quantities of water, oil and gas produced from the affected wellbores. Such relative rate changes may affect the ability of the surface facilities to operate efficiently.
One way to determine expected quantities of produced fluids from each wellbore in each reservoir is to mathematically simulate the performance of each well in each reservoir to be coupled to the surface facilities. Typically this mathematical simulation is performed using a computer program. Such reservoir simulation computer programs are well known in the art. Reservoir simulation programs, however, typically do not include any means to couple the simulation result to a simulation of the operation of surface facilities. Therefore, there is no direct linkage between selective operation of the various wellbores and whether the surface facilities are being operated in an optimal way.
One system that attempts to couple reservoir simulation with surface facility simulation is described in, G. G. Hepguler et al, Integration of a field surface and production network with a reservoir simulator, SPE Computer Appl. vol. 9, p. 88, Society of Petroleum Engineers, Richardson, Tex. (1997). A limitation to the system described in the Hepguler et al reference is that it is unable to generate a corrective action with respect to the surface facilities which may arise out of infeasibility. Infeasibility is defined as the production system operating outside a constraint or limit, for example, defining a maximum allowable water production which is lower than an expected water production from reservoir simulation. Another limitation in the Hepulger et al system is that there is poor convergence in an optimization routine in the system. Other prior art optimization systems are described, for example in M. R. Palke et al, Nonlinear optimization of well production considering gas lift and phase behavior, Proceedings, SPE production operations symposium, p. 341, Society of Petroleum Engineers, Richardson, Tex. (1995). This reference deals primarily with optimizing gas lift systems and does not describe any means for optimizing surface facility use in conjunction with optimizing reservoir production.
A method for optimizing production allocation between wellbores in a reservoir is described in, Zakirov et al, Optmizing reservoir performance by automatic allocation of well rates, Conference Proceedings, 5th Math of Oil Recovery, Europe, p. 375 (1996). The method described in this reference does not deal with optimizing the use of surface facilities in conjunction with optimizing reservoir production.
It is desirable to have a simulation system that can enhance or optimize, both reservoir production and surface facility operation simultaneously, while also being able to assist in isolating and rectifying causes of the production system operating outside constraints.